Powder metallurgy



United States Patent 3,366,479 POWDER METALLURGY Samuel Storchheim,Forest Hills, and Alan Cross, Little Neck, N.Y., assignors to AlloysResearch & Manufacturing Corporation, Woodside, N.Y., a corporation ofDelaware No Drawing. Continuation-impart of application Ser. No.210,072, July 16, 1962. This application Apr. 28, 1965, Ser. No. 454,768

3 Claims. (Cl. 75-214) ABSTRACT OF THE DISCLOSURE A fine grainedmetallic article produced by powder metallurgy from a metal powder suchas aluminum is obtained by (a) coordination of the amount of lubricantin and compaction pressure of a shaped mass of the powder so as toestablish interconnecting porosity, and (b) heating the shaped masshaving interconnecting porosity up to its sintering temperature in amoisture-containing atmosphere at a rate such as to control poreoxidation and form an oxidic internal-skeleton supporting-structure soas to prevent sagging during sintering.

This application is a continuation-in-part of our earlier filedapplication Ser. No. 210,072, filed July 16, 1962, Ser. No. 373,445,filed June 8, 1964, and Ser. No. 398,490, filed Sept. 21, 1964,respectively, all now abandoned.

This invention relates to an improvement in aluminum powder metallurgyand more particularly to a product and a method for obtaining the samecharacterized by high strength isotropic properties.

Powder metallurgy is a very convenient method of obtaining intricatelyshaped objects with a minimum of effort and expense. Broadly speaking,the technique of powder metallurgy involves mixing the particularmetallic components in powder form, inserting the mixture of metallicpowders in a suitable shaped die or mold in order to impart the desiredshape, followed by sintering at a temperature sufficient to causediffusion of the powder particles to form a substantially homogeneousarticle. While the technique of powder metallurgy has been in use "formany years, there are certain metals which can only be treated in thisfashion, if at all, with extreme difficulty. Foremost among thesedifliculty treatable metals is aluminum. A characteristic of aluminum isits tendency to rapidly form an inert oxide coating. Such an oxidecoating on aluminum particles serves to act as an insulator to preventadequate diffusion between adjacent particles. Even in those instanceswhere powder metallurgy techniques have been applied to aluminum, theproducts obtained are characterized by rough surface distortions,non-uniformity of appearance, and very frequently contain blow holesand/or blisters. For these reasons, the powder metallurgy techniqueswith respect to aluminum has never become a commercial reality.

From a theoretical standpoint, if completely pure elemental aluminumwere employed, and the entire operation took place in the completeabsence of any oxidizing substance such as, for example, water vapor,some of the problemsinherent in aluminum powder metallurgy might beobviated, but such conditions are substantially impossible.

Cremer and Cordiano in Technical Publication No.

. containing over 90% 3,365,479 Patented Jam. 30, 1968 1574 of theAmerican Institute of Mining and Metallurgical Engineers (1943)discussed some of the problems in aluminum powder metallurgy, andindicated that for any kind of successful results, the die or moldemployed for shaping the powder mixture would have to be lined with aflat lining of overlapping metal flakes which are applied by suspendingflake powders in carbon tetrachloride and spraying a thin film to form asubstantially impenetrable layer on the die walls. Under somecircumstances a fatty acid or soap was added to the carbontetrachloride. Even with this precaution, Cremer and Cordiano stillfound that it was necessary to employ shaping pressures on the order of50 t.s.i. in order to get good results. This die coating and highpressure shaping was required even if aluminum powder was not the soleconstituent but was present in admixture with other alloying powders.

Other workers such as Goetzel in US. Patent No. 2,155,651 have found itnecessary to use a completely desiccated atmosphere during sintering,which takes place at a temperature of about /3 of the weighted averageof the melting points of the elements present in the mass, atemperature, which from the specific examples given, appears to beintended to maintain sintering below the eutectic temperature.

The as-sintered products can be fabricated to have elongations at breakof at least 5%, when the product has a density equal to or greater thanof the theoretical density and in many instances at lower densities.

Powder metallurgy techniques applied to aluminum in the past aresubstantially incapable of producing regular uniformly appearingspecimens. Accordingly, the products of this invention aredistinguishable from conventional powder metallurgy products by theirregularity, uniformity and even appearances, free from bulges,irregularities and macroscopic holes.

By means of this invention, it is possible to fabricate aluminousproducts by powder metallurgy techniques that will have extremely highultimate tensile strengths, extremely high crush strengths and excellentductility. Contrary to the methods hitherto employed in the powdermetallurgy of aluminous parts, the method of this invention is capableof yielding shaped aluminous parts, by weight of aluminum, having crushstrengths in excess of 8,000 psi. and frequently in excess of 10,000 psiwith a deflection at failure of greater than about 3%. Suchcrush-resistant products are particularly useful as bearings.

The powder meallurgy products of this invention are particularly usefulfor cold forging. For example, if a cylindrical blank prepared inaccordance with this invention is cold forged by conventional means intoa complex shape such as a gear in a closed die, the metal flowsuniformly into the die cavity and all teeth are well formed so that thegear product exhibits dimensional uniformity. In the case of castwrought aluminum compositions employed in this same die to produce agear, the metal flow would be non-uniform and thus certain teeth of thegear would be less well formed than others. The gear in general wouldexhibit obvious dimensional non-uniformity.

Similar superiority of the compositions of this invention is seen inother application. For example, if a cylindrical blank produced inaccordance with the invention were cold impact extruded into athin-walled cylindrical can, the uppermost edges of the can would beobserved to be substantially at right angles to the can withoutprojecting points or waviness. In contrast, conventionally prepared castwrought aluminum of the same chemical composition when extruded in thismethod, would be observed to have a considerable number of cars, i.e.,projecting points, requiring that excessive amounts of material betrimmed from the can during the finishing operation.

In carrying out the process of this invention, particulate aluminum plusa lubricant are admixed with any additional metal powders, ashereinafter described, placed in a suitable die or mold, compressed, ifdesired, depending upon the ultimate density and degree of porosityrequired, and placed in a suitable heating area such as a furnace oroven, and heated rapidly up to the sintering temperature in"the'presence of an inert atmosphere (inert to aluminum at the sinteringtemperature) having a restricted (but not absent) moisture content. Thistemperature is maintained for a itme sufficient to develop optimumsintered and alloyed properties in the product. At the conclusion of thesintering operation, the product is cooled rapidly, or slowly asdesired. Thereafter, any one of the variety of subsequent workingtreatments can be performed.

The aluminum should be in particulate form. The exact shapes of thealuminum particles are relatively unimportant in that they can be in theform of powder, needles, wire clipping, scrap turnings, and the like.However, the use of generally spherical particles are generally lessresponsive to pressure transmission during the compaction stage. Thus,any non-spherical aluminum particulates having major dimensions inexcess of about 3 microns are satisfactory. Smaller aluminum particulateoften present both an explosion hazard and a die filling problem. Themaximum particle size will be determined by the size of the finalproduct, the capacity of the equipment employed and by the necessity ofinsuring a uniform mixture of each of the components. For example, inthe case of products intended as forging blanks for hot and cold forgingand extrusion applications where compacting pressures in excess of 3psi. are to be employed, the aluminum particles should not be too coarseand hence should preferably pass through a mesh sieve. In someinstances, it is possible to employ particles as large as /2 inchaluminum granulated ingots or shot.

Typical commercially available particulate aluminum materials that canbe employed as starting materials in this invention include thefollowing air-atomized types having the specified mesh distribution,expressed in percent by volume.

The final powder mix employed in this invention will contain from to 99%or more of the particulate 'Where a pure aluminum product is desired, ofcourse,

no additional metals need be added to the powder mix. However, thestrength properties as well as other attributes of aluminum can beenhanced, as is well known, by the use of other alloying elements. Thus,for example, a stronger product can be obtained in the case ofanaluminum-copper alloy containing about 5% copper than can be obtainedwith pure aluminum. Accordingly, it is often highly advantageous for thedevelopment of a strong final product to include a small quantity ofparticulate copper in the powder mix. The type of particulate copperemployed is again not critical except that good results are attainedwhen a dense product is desired if a small amount of copper in flakeform is present in the powder mixture. Thus, if the compaction pressureemployed, as will be explained hereinafter, is such as to compress thepowder mixture to a density in excess of about 85% of its theoreticaldensity, then the presence of copper flake serves not only as a sourceof copper to form the alloy but also serves as an auxiliary lubricant toease compaction and ejection problems. For such purposes, from about 1%to 5% by weight of the total powder mixture can be copper flake. Caremust be taken, however, when employing copper flake to use an amountless than that which would cause visible striation in the powdermixture. This is a phenomenon easily observed by the naked eye.Generally speaking, no striation problems arise when up to about 5% byweight of copper flake is employed. Occasionally, greater quantities ofcopper flake, for example, up to 10%, can be employed without seriousstriation problems if the particle size of the aluminum powder is quitesmall.

Conventional copper flake can be employed and will generally range inthickness from 8 X10- inch to 5 X 10* inch with flake area generallyranging from 1.5 10 to 4.9 10 square inch per flake. Commerciallypreferred Mesh Distribution Type A 100 11.0 (percent vol.) 13 4 FlowRate Sec 0 gm Density gm./cc 1.115 1 165 In addition, commerciallyavailable centrifugally spun molten aluminum particulates are available.Typical examples have the following dimensions:

flake range in thickness from 05x10" to 8x10" and in area from 0.15 l0to 4.9x l0 square inch.

Where alloys containing a greater amount of copper are required, orwhere advantage need not be taken of the beneficial properties of thecopper flake, copper powder can be employed as a supplement to or inplace of copper flake. Copper powder, when employed, is generally notemployed in particle size greater than that which would pass through a325 mesh sieve. The large particle-sized copper powders tend to causethe formation of macroscopic holes in the final product. Of course, ifsuch holes are desired in the final product, then larger particle-sizedcopper powder can be employed.

While copper has been described as an alloying additive in the powdermetallurgy mix of this invention, it can be supplemented partially orreplaced completely by other alloying additives including, for example,magnesium, aluminum-magnesium alloys, silicon-aluminum alloys, zinc,brass, or any other conventional aluminum alloying additives. Thesealuminum additives can be in the pre-alloyed form with aluminum or othermetals or can be present as the pure elemental metals. Certain additivessuch as silicon would only be added in the prealloyed form. Whenemployed, they will be present in the particulate form having a particlesize sufiiciently small to pass through a 25 mesh sieve and preferably,to avoid the presence of macroscopic holes, through a 325 mesh sieve.The actual concentrations and proportions of the various alloyingelements are preselected in accordance with the end properties of thefinal product. Again, the use of from about 1 to 5% by weight of thetotal powder mixture of flakes of the alloying additive results infacilitating compaction.

The total amount of alloying additives such as copper and magnesium,including both fiake and powder employed in the powder mixture, canrange from 0 when no alloying additives are present up to about 45% ofthe weight of the mixture. Generally, the best physical properties areobtained when the amount of alloying additives in the mixture is belowabout and preferably within the range from about 1% to about 8% of theweight of the mixture.

The lubricant added to the powder mix should be one that is non-reactivewith any of the components in the mixture and non-oxidizing. Thelubricant can either be a liquid or a powder. Furthermore, the lubricantshould be capable of volatilizing completely at a temperature 'withinthe range of 100 C. and the eutectic temperature of the particularmixture of metals in the powder mix. Preferably, volatilization shouldoccur below about 400 C. This *volatilization must be complete withoutleaving any residue. Organic non-oxidizing materials such as highmolecular weight amides have been found to be most suitable for thispurpose. Saturated hydrocarbons, such as paraflin oil, havingsufficiently high boiling points, are also acceptable. Generally,conventional die lubricants are satisfactory. Representativecommercially available lubricants are Nopcowax and Stearotex. A goodlubricant is one that facilitates the sliding of metal particles pastone another. However, the actual chemical structure of the lubricant isimmaterial as long at it possesses the properties indicated above. Thelubricant serves to aid in dispersing the powders as well as tolubricate during the compressing stage.

The presence of a lubricant in the mixture, in accordance with thisinvention, in addition to its lubricating function, also serves anotherpurpose in the process. Under sintering conditions, volatile impuritiescontained in the starting metals vaporize forming gas pockets within theinterstices of the metal. The gas pressure within the metal builds upwith the continuance of sintering, as the gas expands, causing the metalto take on an uneven shape or, in many instances, the gases are presentin sufficient quantity to break through the surface of the metal causingbulges, holes, and miscellaneous other indentations in a random pattern,thereby detracting from the appearance of the final product and vastlydiminishing the strength of the product in unpredictable fashions. Whenthe lubricant is uniformly dispersed within the mixture, the lubricantvolatilizes rapidly while the powder mixture is being heated to thesintering temperature. The lubricant, now in the gaseous form, escapesfrom the product, forming a series of interconnecting pores or ventports, microscopic in dimension, throughout the metallic structure, atthose places where the original lubricant was mixed in and where the gaspassed through. This interconnecting pore or ventport structure remainsafter the lubricant escapes. As sintering proceeds, and gaseousimpurities are formed during the sintering operation, gas pressurewithin the product is not given an opportunity to build up to causedistortion since as gas forms, it escapes through the interconnectingpores previously formed by the volatilized lubricant.

Where the product is not compressed to any appreciable extent,interconnecting porosity need not be supplied in this faction sincethere will be relatively large spaces at abundant intervals throughoutthe shaped object, which might, for example, he intended for use as afilter. Therefore, the lubricant can be omitted, if desired, in thoseinstances where the pressure applied during shaping is less than about 3t.s.i. For greater shaping pressure, the lubricant is a necessarycomponent of the mixture, in an amount of at least A% and up to about 5%by weight.

While the invention has been described in terms of aluminum beingemployed as the primary metal, problems similar to those encountered inaluminum powder metallurgy are also experienced when magnesium, titaniumor beryllium are the primary metals. Hence, this invention can beadopted to the use of magnesium, titanium or beryllium in place ofaluminum as the primary metal. Also applicable to processing inaccordance with this invention are other metallic elements whose oxidesare not reducible by hydrogen at the melting point of the element, suchas zinc and lead.

Particular combinations of metals are useful for particular purposes.For example, aluminum-copper alloys form particularly strong productswhen produced in accordance with this invention. In some instances,however, as when cooper content exceeds about 2%, the compact undergoesshrinkage and becomes distorted during sintering at the elevatedtemperatures employed. The addition of particulate magnesium to thepowder mixture, either in the elemental form or as a master alloy withaluminum, in an appropriate amount, serves to reduce or minimize, if notcompletely eliminate such shrinkage and distortion. In addition, themagnesium adds other beneficial advantages including a speed up ofsintering time, reduction in product discloroation, increased strengthand toughness of the final product, and increased posthardeningresponse. When the amount of magnesium is increased beyond the amountthat serves to eliminate shrinkage, an expansion effect may be obtained,which is useful for some purposes. For the magnesium to serve to reducedistortion, the powder mixture will contain at least about 2% copper andgenerally not more than about 8% copper, and magnesium in an amount ofat least about 0.05%. Frequently, an amount of magnesium in excess ofabout 1% is not necessary to control shrinkage but additional amounts,up to about 10% are sometimes required. For some applications, up to 20%magnesium is desirable.

Shrinkage is also noted frequently upon sintering when the alloyingadditive mixed with aluminum is a zinccopper alloy such as brass.Silicon also has the effect of counterbalancing shrinkage caused bycopper and brass in aluminum systems, but is not by any means aselfective in this respect as magnesium.

Some or all of the beneficial effect of magnesium on the aluminum-coppersystem is lost if the powder mixture contains certain additional metalssuch as manganese, nickel and iron. These latter metals tend to detractfrom the tensile strength improvement imparted by the magnesium andtherefore are best omitted from the aluminumcopper-magnesium systemunless some special effects are needed. In general, in thealuminum-copper-magnesium system, impurities should be minimized. Ofcourse, when aluminum-magnesium master alloys are employed as the sourceof magnesium, impurities in such alloys are best avoided or in any eventkept below about 1% by weight based upon the master alloy.

After the lubricant and the various metals are mixed,

they should be thoroughly blended to insure a uniform dispersion. Themethod of blending is essentially immaterial as is the time of blending.As an example, blending may take place for a period of some 15 to 60minutes in a double-cone blender or in other suitable devices.

The mixed powder is placed in a suitable mold or die having theparticular shape desired of the sintered product. Where a bearing is tobe produced, the compacting can be effected by means of a die capable ofproducing the desired outside diameter, into which a core rod isinserted to establish the required inside diameter. To maintain uniformdensification, equal top and bottom punch movements are used to impart apressure of from about 3 to 40 t.s.i. and thus obtain the desireddensity, which can range anywhere from as low as 60% or less oftheoretical to over 95% of theoretical. Upon release of pressure, theshaped bearing has sufiicient integrity. If the final product is desiredto be porous for use in filter applications and the like, little or nocompaction is necessary. The degree of porosity of the final product canbe controlled by the compaction pressure that is applied. In general, ifthe product is to be porous at all, the compaction pressure must bebelow 5 t.s.i. In the method of this invention, satisfactory porousproducts can be obtained even in the absence of any compaction and formany purposes, no compaction at all is preferred.

The actual compaction pressure employed depends upon the density desiredof the final product. A porous product is generally one, which aftercompaction, exhibits a density of up to 85% of the theoretical densityfor the particular mixture. Theoretical density is calculated by takinginto consideration the density and volumetric con centration of each ofthe individual components present in the mixture before treatment.

The interconnecting porosity, or ventport structure, which constitutesan important feature of this invention, generally ceases to exist at adensity greater than about 98% of the theoretical density. Accordingly,the compaction pressure employed in this invention should be such as tonot exceed a theoretical density of 98%. For most purposes, thecompaction pressure will be below 40 t.s.i. and rarely will thecompaction pressure even reach 30 t.s.i. For high strength,substantially non-porous structural parts, the pressures employed aresuch as to produce a density in the compacted product ranging from about85% to 98% of the theoretical density.

Thus there can be seen that for porous products, the compaction pressureemployed will range from up to about t.s.i. while for high strength,substantially nonporous products, the compaction pressure will rangefrom about 5 up to about 30 t.s.i.

Once the appropriate density is achieved, the compacted mixture must beheated rapidly to the sintering temperature. The rate of heating is veryimportant. The sintering temperature itself is selected by a process oftrial and error for each particular system. The particular sinteringtemperature adequate for any given system is a temperature above theeutectic temperature but below the point of general fusion of themetallic components at the concentration employed and preferably belowthe fusion temperature of the primary metal. For purposes of thisspecification, the term eutectic temperature is intended to be inclusiveof what is normally referred to in metallurgy terminology as theeutectic temperature, the peritectic temperature, or if no binary orpolymetal eutectic effect is observed, the fusion temperature of thelowest melting point metal present. If only one metallic element ispresent, the eutectic temperature is herein arbitrarily defined as atemperature 50 C. below its melting 7 point. For many alloys, optimumconditions are achieved in the range between the solidus temperature andthe liquidus temperature.

Within this range of temperatures, generally encompassing a spread offrom 70 C. to 200 C., the optimum temperature is that where theinterditfusion or self-diffusion of the alloying constitutents oraluminum particulates, respectively, is rapid enough such that adequateproperties are attained in a commercially acceptable time period. Foreach composition, the particular optimum temperature will vary to someextent within the stated limits and thus must be determinedindividually. This can be done conveniently by preparing several smallsamples for a given composition and sintering each in accordance withthis invention, using temperature approximately 20 degrees apart withinthe applicable range and determining from the ultimate tensile strengthof the several samples what would be the optimum temperature range.There would normally be about a 15 C. to 20 C. optimum range for eachcomposition. For aluminum-magnesium and aluminum-copper systems, it isgenerally found that the sintering temperature exceeds about 600 C.

The range of heating to the sintering temperature must be as fast aspossible short of causing blistering due to too rapid gas build-up. Arate of heating of from 20 C. to about 600 C. per minute is employed.The minimum effective heat-up rate within this range will depend uponthe environment, the composition of the particular mixture and thedegree of compaction. Once established for a given mixture, the rate ofheating can be maintained for subsequent runs with this mixture. Thefaster the rate of heating, the better the strength properties of thefinal product. Offset against this is the tendency of the product toblister or to degrade in properties when the rate is too high.Accordingly, the optimum heat-up rate for any given powder compositionmust be determined by trial and error. In most cases, the minimumeffective rate of heating will be at least about 40 C. per minute,preferably 50 C. per minute, and in many instances, a higher rate ofheating is required. A guide for use in determining minimum heat-up ratein a given instance is discussed below.

From the scientific standpoint, it appears that the heatup rate employedmust be at least greater than the rate of oxidation under the heatingconditions. However, the actual rate of oxidation is difficult tocompute so that trial and error is the most satisfactory method ofachieving optimum heat-up rates.

In considering heat-up rate, it is not the rate at which the furnace orother heating means reaches the sintering temperatures that isimportant, but instead, it is the actual temperature of the sample. Thiscan easily be ascertained by conventional means through the insertion ofa thermocouple in the sample. Preferably, the temperature conditionswithin the furnace and the thickness of the sample should be such thatthe temperature through the sample is approximately the same.

While it is desirable that the heat-up rate be rapid from roomtemperature to the sintering temperature, it is only essential that thisrapid rate be employed starting from the volatilization temperature ofthe lubricant or 400 C. whichever is the lower.

A particularly important aspect of the use of magnesium is the fact thatwhen the magnesium content of the aluminum-copper-magnesium powdermixture does not exceed about 0.3%, a slower heat-up rate can betolerated. Thus when the magnesium content of the mixture is between0.05% and 0.3%, the heat-up rate frequently can be as low as 5 C. perminute. In many cases better results are obtained with faster heat-uprates, in

excess of 20 C. per minute. However, where economic or otherconsiderations dictate a slower heat-up rate, these low magnesiummixtures can be fabricated into products having elongations of at least5% at the slower rate. Once the magnesium content is in excess of about0.3%, the heat-up rate must be at least 20 C. per minute as describedabove. Only in a very few instances, particularly when very fineparticle size aluminum powder is employed can the magnesium content beas high as about .5 at the slower heat-up rate while still obtainingmar- '9 ginally acceptable properties. For aluminum-magnesium mixtures,in the absence of copper, the heat-up rate should be in excess of 20 C.per minute, regardless of the magnesium content.

The sintering atmosphere must be carefully controlled. The atmospheremust be inert to the components present and non-oxidizing in nature. Areducing atmosphere such as an atmosphere of hydrogen is useful as anadded precaution in many instances, but is not indispenable. Suchreducing atmospheres are included within the scope of the term inertatmosphere since they are only precautionary and not intended toactually take part in the process. Other inert atmospheres such asatmospheres of helium, argon, neon, xenon and krypton can also be used.Nitrogen-containing inert atmospheres such as dissociated ammonia can beused, but generally are not completely satisfactory unless magnesium isone of the metallic components employed.

The moisture content of the atmosphere must be carefully controlled. Theatmosphere must be semi-dry but not moisture-free. It is important tohave some moisture present in the sintering atmosphere since in thepresent invention, the presence of a limited amount of moisture appearsto have an effect on causing the formation of a finer-grained-strongerproduct. This is a beneficial effect of the invention sincemoisture-free atmospheres are expensive and diflicult to maintain. Themoisture content of the atmosphere employed during the sintering step ofthis invention should be such that the atmosphere in the environment ofthe sintered material would have a dew point ranging from -80 F. to 10F. By dew point, it is meant that temperature at which the gas in thevicinity of the sintering metal would have to be cooled before watervapor present in the gas would condense.

If the atmosphere has a higher moisture content than a dew point ofabout 10 F., there is a greater tendency toward oxidation of the sampleduring sintering which consequently greatly reduced strength properties.Dew points dryer than 80 F. are uneconomic and are best avoided.

It is believed that the presence of a limited amount of moisture resultsin the formation of an internal skeleton supporting structure whichimparts strength at the sintering temperatures and prevents the sagging,and in some cases, even the melting that would normally occur at thesintering conditions. This phenomenon is of greatest importance inconnection with relaitvely heavy objects since the sintering of heavyobjects under conditions fostering formation of a liquid phase, evenbriefly, frequently leads to sagging in the prior art methods. The exactpoint at which such sagging occurs is difficult to predict. Accordingly,the use of the semi-dry atmosphere in accordance with this inventionobviates the problem,

The preferred range of dew points is between 20 F. and 60 F. When thepowder composition contains small quantities of metals such asmagnesium, the dew point can occasionally be raised to be above 10 F.without deletrious results, but even then, dew points much above +lwould not be desired.

The minimum heat-up rate required for this invention varies with theconditions of the sintering environments. As a guide to determining theminimum heat-up rate, an equation has been developed for a pure powderedaluminum-lubricant mixture. Use of this equation results in the minimumheat-up rate possible for a given system. The optimum heat-up rate forthe particular system may lie somewhat above this value predicted by theequation, but not below it. In any case, the minimum heat-up rate willnot be less than 20 C./min.

Using a ten-minute sintering time, a heat-up rate of 45 C. per minuteand a dew point in the furnace atmosphere of 20 F., the followingsintering temperatures were employed for particular aluminum-coppermixtures varying weight percent of copper flake and compressed to adensity of of theoretical density with /2 of an organic lubricant.

Weight percent copper Sintering temperature, C.

It is significant to note that in accordance with this invention, thereis no need to separately lubricate the forming die or mold in order toprevent lamination to the mold wall, a procedure which has beenindispensable in many other processes.

After sintering, the method of cooling the sintered product isrelatively immaterial. It can be slowly cooled to room temperature byallowing it to remain in the furnace or it can be rapidly cooled byquenching or the like.

In practice, the process of this invention can be adapted for operationon a continuous basis. After a large batch of powder is mixed, means canbe provided for automatically ejecting predetermined quantities intosuitable molds, compacting, if desired, and placing the molded productwith or without the mold on a conveyor belt and passing the conveyorbelt through a series of temperature zones in which, successively, thelubricant is allowed to volatilize, rapid heat-up is promoted in a hightemperature zone, the sintering temperature is maintained and thereafterthe sample is passed through a cooling or aging zone. The same resultscould, of course, be achieved in a one-zone furnace.

An actual embodiment of a furnace capable of use in the work describedherein has a straight mufiie measuring 5 x 4" in cross-section, and isprovided with a set of bafiles at inlet and outlet, the bafflesconsisting of a set of three curtains made of thin stainless steelstrip. The atmosphere used is tank hydrogen with a dew point of 80 R,which is fed into the mufile through an inlet. With such an atmospheresupply, an average dew point of 20 F. or better can be maintained withinthe furnace muflle. The actual value maintained will depend upon thehydrogen flow rate used.

The furnace is 5 feet in over-all length, and is provided with two setsof silicon carbide heating element arranged above and below the mufile.Parts are carried through the muffle on a continuous belt mounted onpulleys. Three individually controlled heating zones are available, andthe power input to each zone may be separately controlled. The centerzone has, in addition, separate control of power input to the elementslocated at the top and bottom of the muffle. This arrangement provides auniform sintering zone 21 inches in length, and allows for a maximum ofabout 8 minutes sintering time when belt speed is adjusted to provide aheat-up rate of 50 C. per minute.

The following examples represent, in the opinion of the inventors, thebest mode of carrying out their invention.

Example I.-(Self-lubricating bearings) A mixture consisting of Type Aaluminum powder containing additives of the type and quantity shown inTable II below, is mixed for /2 hour and is compacted in the form of /2"ID x OD x /2" long sleeves, to a density of 77% to 78% of theoreticaldensity. The green compacts are then continuously sintered in a hydrogenatmosphere using a belt-type sintering furnace. Belt speed is adjustedso as to obtain a heat-up rate to sintering temperature of approximatelyC. per minute. The dew point in the furnace was 20 F.

The sintered parts are found to he possessed of excellent crushstrength, on the order of 1,000 to 13,000 p.s.i. for the aluminum-4weight percent copper alloy, and 35,000 p.s.i. for the aluminum-4 weightpercent copper-0.6 weight percent magnesium alloy, as shown by the dataof Table I. When sized and oil-impregnated, such parts ex- Examples 3and 4 An aluminum powder, all of which passed through a 100 mesh sieveand containing 35.6 weight percent of ma- The lubricant was a highmolecular weight organic amide.

terial capable of passing through a 325 mesh sieve was 5 mixed forminutes with 4% by weight of copper flake,

TABLE I.-FABRICATION CONDITIONS AND PROPERTIES OF POROUS ALUMINUM COPPERAND ALUMINUM-COPPER-MAGNESIUM ALLOY BEARINGS Bearing Composition Type AAluminum Powder, 2 wlo 100/200 mesh copper powder,

2 w/o copper flake, 2 w/o lubricant. 1

Type A Aluminum Powder, 2 wlo 100/200 mesh Cu 2 w/o copper flake, 0.6w/o 325 mesh magnesium powder, 2 wlo lubricant.

Sintering furnace used Furnace temperature F 1 Sinteriug time Densitybefore sintering, percent Density after sintering, percent Dimensionsbefore sintering Optimum dimensions after sintering.

Interconnecting porosity, as sintered, percent.

Crush strength at optimum sintered dimensions, p.s.i.

Continuous belt type with straight through muffle. ,100 .049 f.p.m. beltspeed corresponding to 20.4 minutes in a 1 foot sintering zone.

.753 O .4975 ID x .4931ong Top: .757 OD to .757 0D, .5005 ID to .5000 IDx 0.502 long. Bottom: .757 OD to .756 OD,

.4995 ID to .5000 ID. About 20 Continuous belt type with Hump mufifle.

1,120. 0.293 f.p.m. belt speed corresponding to 3.4 minutes in a 1 footsintering .753 x .4975 ID X .539 long.

Top: .753 OD to .7540 OD, .4980 ID to .4985 ID x 0.548 long. Bottom:.7550 OD 130 .7555 OD, .499 ID to Approx. 35,000.

Example 2.(High-density structural parts) 0.3% by weight of 325 meshhelium-atomized magnesium and 0.5% by weight of Nopcowax (a fatty amide)A mixture consisting of Type A aluminum powder, copper flake in amountsup to 6 w/o, and /2 w/o lubricant,

obtain a heat-up 4 lubricant. The mixture was divided into several partsand each part was compacted in a shaped die to 95% of its theoreticaldensity, using a compacting pressure of 20 t.s.i. the compressedproducts being in the shape of A1" x /2 x 4" bars. Each of the bars wasthen heated separately to the sintering temperature at a rate givenbelow in an atmosphere of dissociated ammonia having a dew point of 20F. to 30" F. in the sintering zone, sintered for a period of time, andthen cooled. The physical properties of the sintered material are setout below, illustrating the very significant effect of heat-up rate andsintering condi- Table II below, ultimate strength and elongationvarying tions on ultimate properties.

Dew Point Time Maximum Ultimate Elongation Example C./Min. of FurnaceAbove Temperature, Tlme at Maximum Tensile Percent Atmosphere, 600 C C.Temp., Min. Strength, in 1' 1 Min. p.s.i.

3 89 -20 to 30 4 646 Less than 1 minute 28, 000 4 59 21 to 27 16 654 do22,600

from 10,1000 p.s.i. and 35% at 0 w/o copper, to 52,000 Example 5 p.s.i.and 13% at 6 w/o copper.

TABLE II.MECHANICAL PROPERTIES OF SOLUTION TREATED AND AGED SINTEREDALUMINUM PREPARED FROM TYPE A ALUMINUM POWDER CONTAINING VARY- 5 INGQUANTITIES OF COPPER FLAKE An aluminum powder (Alcoa 120) was mixed with4% copper flake and /2% Nopcowax and a quantity of magnesium powder asset forth below and compressed to of theretical density, using acompacting pressure of approximately 20 t.s.i., the compressed productsbeing in the shape of V8" x /2" x 4" bars. Each of the bars was thenheated separately in a hump furnace to the sintering temperature ofaproximately 645 C. to 650 C. in an atmosphere of dissociated ammoniahaving a dew point in the vicinty of the sintering product ofapproximately 44 F. Each sample was maintained at the sinteringtemperature for about 5 minutes and then re 7 moved and allowed to coolat room temperature. Physical more After aging the samples at roomtemperature for 48 hours, the following physical properties wereobserved:

Heat-up Rate Magnesium C/min. from Elongation per- Ultimate Content, 400C. to cent in 1 inch tensile Strength, percent Siutering p.s.i.

Temperature The foregoing illustrates the unusual efiect of a very smallquantity of magnesium on aluminum-copper systems and further shows thatat magnesium concentrations below about 0.3%, a heat-up rate of lessthan C. per minute can be employed although appreciably betterelongations are obtained at high heat-up rates.

Example 6 An aluminum powder (Alcoa 120) was mixed with /2% Nopcowax andwith varying quantities of magnesium powder as set forth below andcompressed to 95% of theoretical density using the necessary compactingpressure, less than 30 t.s.i., the compressed products being in theshape of Ms" x /2" x 4" bars. Each of the bars was then heatedseparately in a hump furnace to the sintering temperature ofapproximately 645 C. to 650 C. An atmosphere of dissociated ammonia wasmaintained in the furnace by passing dissociated ammonia through at arate of 300 standard cubic feet per hour. The dew point in the vicinityof the sintering product was approximately 58.5 F. Each sample wasmaintained at the sintering temperature for about 3 minutes and thenremoved and allowed to cool at room temperature. Physical propertiesobserved immediately after sintering and cooling were as follows:

Heat-up Rate Magnesium C/min. from Elongation per- Ultimate Content, 400C. to cent in 1 inch tensile Strength, percent Sintering p.s.i.

Temperature 0. 55 8. 5 9, 800 0. 72.5 17 10,900 0. 87 16 12, 900 0. 92.5 27 12, 200 0. 55 6 8, 900 0. 72.5 10 10, 000 (J. 87 14 12, 800 0. 92.5 19 12, 400 l. 55 2. 5 6,000 1. 72 5 5 8, 700 l. 87 5 10, 000 1. 92.510 14,000 2. 55 3. 5 8,400 2. 72. 5 2 6, 400 2. 87 3.5 11, 100 2. 92. 57 14, 300

' tents.

What is claimed is:

1. In the method of preparing a cohesive and finely grained metallicarticle containing more than 50% by weight of a primary metal whoseoxide is not reducible by hydrogen at the melting point of the metalwherein the primary metal in particulate form is shaped to achieve thedesired configuration and density, and the shaped mass is heated tosintering temperature, the improvement which comprises: (a) shaping themass to a density not exceeding 98% of the theoretical densitycharacterized by interconnecting porosity when any lubricant therein hasbeen volatilized by heating the mass to 400 C. by using an amount of thelubricant ranging from 0 to 5% by weight and an amount of compactionpressure ranging from 0 to 30 t.s.i., and (b) maintaining the shapedmetal in a moisture-containing atmosphere having a dew point range of F.to -20 F. while heating the shaped metal having interconnecting porosityat a rate of at least 20 C. per minute up to the sintering temperatureof the metal.

2. The method according to claim I in which the primary metal isaluminum.

3. The method according to claim 1 in which the metallic articleconsists essentially of a aluminum.

References Cited UNITED STATES PATENTS 2,228,600 1/1941 Hardy 2l42,287,251 6/1942 Jones. 3,232,754 2/ 1966 Storchheim 7520O X CARL D.QUARFORTH, Primary Examiner. L. DEWAYNE RUTLEDGE, Examiner. R. L,GRUDZIECKI, Assistant Examiner.

